1. Field of the Invention
The invention relates to an industrial robot, in particular, to a device for detecting the limits of the rotational motion of a rotating member in an industrial robot.
2. Description of the Related Art
An industrial robot has a robot arm, and a robot wrist is provided at the distal end of the robot arm. An end effector, such as a robot hand, is mounted onto the robot wrist. The robot arm has a plurality of articulated portions which include a fixed member and a rotating member which rotates relative to the fixed member. The end effector is moved along a desired path due to the relative motion between the rotational and fixed members according to a program in a robot control unit. In order to prevent the robot arm moving beyond a predetermined limit, a device for detecting the limits of rotational motion, such as a limit switch, is provided at the articulated portion.
Referring to FIG. 10, a prior art device for detecting the limits of rotational motion is illustrated. The device is provided at the articulated portion. In FIG. 10, a fixed member 46 has dogs 48a and 48b. On the other hand, a rotating member (not shown) has a limit switch 44 electrically connected to a robot control unit (not shown). The limit switch 44 may be a conventional plunger type. The rotating member can rotate relative to the fixed member 46 about an a'-axis.
At the home position, indicated by O', the limit switch 44 is drawn with solid lines. When the rotating member rotates in the counterclockwise direction from the home position, the limit switch 44 also rotates in the counterclockwise direction therewith. When the limit switch 44 rotates to the limit of the rotational motion indicated by X, the limit switch 44 contacts the dog 48a. The limit switch 44 detects that the rotating member has rotated to the limit and sends a signal to the robot control unit. Upon receiving the signal, the robot control unit stops the rotating member.
On the other hand, when the rotating member rotates in the clockwise direction from the home position O', the limit switch 44 also rotates in the clockwise direction therewith. When the limit switch 44 rotates to the limit of the rotational motion shown by Y, the limit switch 44 contacts the dog 48b. The limit switch 44 detects that the rotating member has rotated to the other limit and sends a signal to the robot control unit. Upon receiving the signal, the robot control unit stops the rotating member.
Thus, the limits of the rotational motion are detected by the contact between the limit switch 44 and the dogs 48a or 48b, and this stops the rotating member of the robot moving beyond the limit of the rotational motion. In FIG. 10, the total rotational movement possible is the rotational angle of 162 degrees in the clockwise and in the counterclockwise directions from the home position. Thus, the rotating member can rotate within the range of 324 degrees about the a'-axis.
As the motion of the rotating member is limited to within the range of 324 degrees about the a'-axis by the device shown in FIG. 10. That is, the prior art device does not allow an angular range of more than 360 degrees. Thus, the robot with the prior art device has a problem in that, sometimes, the robot cannot move along a desired path and it takes long time to teach the robot arm. For that reason, some robots are not provided with a device for limiting the motion of the robot arm. This is a problem from the view point of safety around the robot.